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Aviation History
1928
1928 - 0808.PDF
SUPPLEMENT TOFLIGHT 64 AUGUST 30, 1928 THE AIRCRAFT ENGINEER about 0-015 in. It must however play an important part in giving rigidity and strength to the structure, for one cannot imagine such light angles as these substituted for the built-up members of the fuselage of the " Bristol " exhibit without incurring considerable loss in strength. The main difference between the " Bristol" and Wibault constructions arises from the assumption that the fairing and covering of the " Bristol " machine takes no load, while the covering on the Wibault machine must be assumed to stabilize the primary members. One cannot conceive that simple duralumin sections used in the elementary manner seen here could, unsupported, sustain anything but very low stresses without failure. The metal-covered Wibault wings are, if anything, even simpler in construction than the fuselage, the method of securing spars, ribs and covering is shown in Fig. 10. A spar consists simply of a flat web, angles being secured to the edges of the web. The ribs consist of angles, having one edge riveted to the spar flanges, the upper edge being shaped to the contour of the aerofoil. The covering is secured in the manner shown in the sketch. Many people believe that metal covered surfaces involve greater complication and increase in manufacturing costs, but judging from the Wibault wing and a small metal wing of Rohrbach design also on view, the reverse would appear to be the case. As most readers will know, the forward and aft portions of the Rohrbach wing are detachable, only the centre portion supplying the moment of resistance. This centre portion has much in common with the Wibault wing. The two spars are similar except that in the Rohrbach design the web is liberally lightened along its length (see Fig. 11) : the triangular holes probably assist in assembly. Where the covering plates abut, a bulkhead consisting of the usual fore and aft lightened flat plate having four angles attached is inserted internally, this is riveted to the skin. Longi- tudinal corrugation is dispensed with, but stout channels having inwardly-turned lips are secured to the skin just aft of the front spar and just forward of the rear spar. The spar webs are also reinforced at intervals with vertical or diagonal stiffeners and the skin has also " occasional " stiffeners. Apart from the longitudinal channels additional stiffness against bending is obtained by turning up the edge of one of the cover plates. A line of joining lies midway between and parallel to the spars. This is a lap joint and the under plate is simply left long, then turned up at right angles and flattened again where the transverse bulkheads cross. It would be impossible to have anything simpler than this. We pass quickly over the Junkers exhibit, its corrugated wings, body empennage, etc., being familiar to most readers. A full description of its design and manufacture is given in the Aeronautical Journal for September, 1923. Returning to fabric covered wings, etc., ease of assembly seemed in general to be the keynote. In wings having rectangular spars, the ribs are fitted with plates having similar rectangular holes. After the ribs are slipped into position endwise movement is prevented by numerous light- gauge wires running the length of the wing. The ribs on the Nieuport are connected to the box spars by suitable angles riveted to brackets secured to the apar webs. A similar method, except that angle brackets are secured to the spar flanges, is adopted for the Caudron double-channelled spars. Mo further comment on these fixings is necessary beyond this, that the inter-connection of open-type girder members by means of suitably bent plate fittings is always a simple , matter ; the only thing to watch is that the riveting points are easily accessible. Reference has already been made to the apparently rather expensive Nieuport ribs. The economy of construction of the Nieuport wing, however, allows of the incurring of extra expense in rib making. The ribs are spaced about three times as far apart as in other wings of about the same size ; for the prevention of fabric sag and other possible troubles a network of curved channelling is used. These subsidiary members spread from wing tip to outer tank ribs, the latter having flat plate webs and ordinary rib flanges with the web heavily reinforced, and from the inner tank ribs to the wing root, also from leading edge to trailing edge. Fig. 12 shows a small portion of the construction, the " stringers" on the underside being omitted. Where these subsidiary members cross each other or cross the main ribs distance pieces are introduced, the connection being made by rivets. This arrangement gave the impression of lightness, the large loads carried by the main ribs make them really economical on a weight-strength basis. The large Y struts used on this machine, joining the undercarriage to the top wing, incidentally passing through and supporting the subsidiary bottom wing, are pressed parts. These members are of the order of 10 ft. or 12 ft. long and only production on a large scale could warrant the necessary tool making for such a job. The writer does not know how many of these machines have been ordered or are on order by the French Govern- ment, but large numbers of what appeared to be similar aeroplanes were seen flying at the Vincennes Pageant. Constructions involving a single large built-up duralumin interplane strut are favoured by French designers, but usually the centre portion is made from two parallel parts riveted together. The ends are separately pressed parts, as exemplified on the Bleriot and Breguet machines. While these strut end pressings must be of fairly involved shape in order to connect the two spars in the upper and lower wings with the one central strut, they are of great simplicity compared with the pressed fitting used on the Dewoitine undercarriage. The production of large duralumin pressings and forgings has evidentally received great attention in France, and the aircraft constructors are fortunate in having this highly-specialised trade at their command. An examina- tion of the stands of the Fonderies Montupet and the Forges de Foulain gave much information as to the variety of castings, forgings and pressings which these firms offer to the French aircraft industry. On the Amiot S.E.C.M. products, a very large number of pressings are used. I believe I am correct in saying that this firm were originally press tool makers, and turned to aviation as a side line ; if this is so, it explains why these complicated pressings are used. The Italian and Czecko-Slovakian exhibits were, as regards main members, entirely of timber. Generally, throughout the exhibits, hollow rivets were extensively used, also solid rivets seemed largely to have taken the place of eyelets. Most of the simple fittings on the square longerons were riveted in place ; bolting would appear to be a simpler method of fixing, nevertheless bolts were used very extensively in certain cases, instances being the attachment of fittings to rectangular spars in wings. It was further noted that where rectangular solid-drawn spars were used, double internal wing bracing was always employed, see Fig. 5, spaced as widely apart as the depth of the spars permitted. Workmanship, generally, was of a very high order, particu- larly tinsmiths' work. Some beautiful cowling and fairing was shown, but it was not always obvious how these parts could be quickly detached. In English construction, great rigidity of the tail portion of an aeroplane is required, for example, if, when a machine stands on the ground it is possible to get any degree of angular movement by pushing up and down on the tail plane tip, the whole structure at once falls under suspicion, no matter how steady the tail is in flight. From the knife-edge shape of some of the fuselage tails exhibited, it appeared that this rigidity would be lacking in many of the exhibits. Also the complete absence of protecting coating from the duralumin structures was very noticeable. It is not a very expensive matter making an aeroplane reasonably corrosion proof, and if any length of life of the structure is required, such protec- tion is of course imperative. A Correction Lines 14 and 15 of right hand column on p. 54 of July 19 issue should read : . . . . Fig. 8A, 8B, and 8c, part A r2 o^ = r2 a2 = r, a, etc. 748i
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